const payload = {
      address: thisCart.dom.address.innerHTML,
      totalNumber: thisCart.totalNumber,
      subtotalPrice: thisCart.subtotalPrice,
      deliveryFree: thisCart.deliveryFree,
      totalPrice: thisCart.totalPrice,
      products: []
    };

Powinno być [] zamiast {}

I druga rzecz ważniejsza, odpal sobie ten kod:

  for (let product in thisCart.products) {
      console.log(product);
      payload.products.push(thisCart.products[product].getData());
    }

Generalnie chodzi mi o console.log